Fix rules's command contains for-update

Hi hackers,

I run the following sql on the latest master branch (head commit:
6f0e19005) of Postgres:

```sql

gpadmin=# create table t(c int);
CREATE TABLE
gpadmin=# create rule myrule as on insert to t do instead select * from t
for update;
CREATE RULE
gpadmin=# insert into t values (1);
psql: ERROR: no relation entry for relid 1

```

It throws an error.

After some investigation, I found that:

1. in the function `transformRuleStmt`, it creates a new ParseState
`sub_pstate` to transform
actions. And in this `sub_pstate`, it is initially contains two
rangetblentry, "old" and "new".

2. in the function `transformSelectStmt`, it will invoke
`transformLockingClause` to handle `for update`. And it loops all the
entries in rtables.

I think for a CreateRuleStmt, its command part if is a select-for-update
statement, the for-update clause should skip the two "new", "old"
RangeTblEntry.

How to fix this:
1. forbid the syntax: rule's command cannot be a select-for-update
2. skip new and old: I have a patch to show this idea, please see the
attachment.

Any thoughts? Thanks!

Best Regards,
Zhenghua Lyu